This invention is directed to an improved process of preparing an aluminum workpiece for the subsequent electrolytic application thereon of a nickel coating.
Recently, interest has increased in the utilization of aluminum in automotive applications in exposed areas, such as bumpers and the like, which has required re-evaluation of various methods of applying nickel-chromium coatings onto an aluminum surface. Due to the tenacious natural oxide which forms on aluminum, special preparations are usually required for the aluminum surface prior to electrolytic plating to insure good adhesion between the aluminum substrate and the nickel-chromium coating.
Heretofore, several methods have been employed to prepare an aluminum surface for nickel plating. One method has been first to treat the aluminum surface with a zincate solution and thereby to deposit a thin layer of zinc onto the aluminum and then to electrodeposit a copper containing metal layer onto the zinc coating from an aqueous cyanide solution containing copper. Another method, similar to the first, involves plating a bronze layer directly onto the aluminum surface from an aqueous cyanide solution containing copper and tin. Although the above methods provide excellent adhesion between the aluminum substrate and the subsequent electrodeposited metal layers, they have the rather serious drawback of employing a cyanide solution which is both difficult and expensive to treat for disposal. Moreover, should the plating be penetrated in service, severe galvanic corrosion can occur due to the large potential difference between the copper and the zinc containing interlayer, or between the copper layer and the aluminum substrate.
The third method generally involves anodizing the aluminum workpiece in an aqueous phosphoric acid electrolyte, and then electroplating one or more layers of nickel. The phosphoric acid anodizing pretreatment method poses a significantly less severe disposal problem than the cyanide pretreatment, but this anodizing process is suitable for only certain aluminum alloys and is particularly ineffective in providing good adhesion with aluminum alloys having low levels (e.g., less than 0.30% by weight, particularly less than 0.20% by weight) of insoluble intermetallic particles, such as iron, manganese, chromium and the like. Other acidic electrolytes, such as sulfuric acid and oxalic acid can be employed, but phosphoric acid is preferred because of the improved adhesion and corrosion resistance provided. For an excellent discussion of this pretreatment method, see "Nickel-Chromium Plating Upon Anodized Aluminum; "Tech. Proc. AES, 98, 52-9 (1961).
For purposes of appearance and additional protection, the nickel plating is usually followed by the electroplating of a thin layer of chromium. Most conventional chrome plating processes comprise electrolytic treatment in an aqueous chromic acid solution containing a small amount of catalyst, such as a sulfate or a fluoride.
Aluminum alloy 7046 (Aluminum Association alloy designation) is presently a prime candidate for utilization as automotive bumper stock. However, because this alloy has low levels of insoluble intermetallic particles, conventional phosphoric acid anodizing pretreatments will not form a suitable base for an adherent nickel-chromium plate.
Against this background, the present invention was developed.